Improved dosing accuracy, backed by tracking, minimizes ingredient waste and boosts profit  

1. Understand the Range of the Dosing Specification

When it comes to feeding and dosing, everything starts with the rate specification. It’s a min/max ingredient level, expressed as a percentage rate of ingredient/resin rate for extrusion (3% of 500 lbs./hr., for example), or percentage rate of ingredient/total shot size (e.g., 4% of 8.2 oz.) per injection molding cycle. Specifications often provide a setpoint – a sweet spot for each ingredient – as well as a plus-minus allowance to account for slight variations, so a rate might specify 3% ± 0.3% of a 500 lbs./hr. ingredient of resin, for example. 

Note that in the latter example, the specification setpoint, 3% ± 0.3% of 500 lbs./hr., allows for a substantial plus or minus variation in ingredient dosing. The high-end ingredient dose allowed is 3.3%, while the allowable low-end dose is 2.7%. And the difference between the high- and low-end doses represents more than twenty percent (22.2% to be exact) of the specified dose. As you might expect, there’s a lot of profit in that difference.  

So, if your goal is to choose the most accurate feeding/dosing system, the key is to select the one that delivers a smooth, accurate feed while minimizing dosing variations. A feeding system that minimizes variations enables you to manage your ingredient dosing rate more precisely – it enables you to operate at the low end of your ingredient dosing specification. So, for example, the right feeder will give you – or your operators – the assurance and the flexibility to run closer to the low-end 2.7% dose, rather than feeling like you have to “play it safe” by boosting ingredient levels to make sure you’re “covered.” Of course, running accurately at the low end of the spec minimizes ingredient usage and reduces material and production costs.   

2. Choose the Right Dosing Tool

Step 2 involves selecting the right feeding tool to use. Base this selection on required ingredient rates and an understanding of the flow characteristics and concentrations of the ingredients you are using. Ingredients are typically dispensed in carrier resins and may be contained in pellets, micro-pellets, powders, or other forms in a specified concentration (e.g., 6g/pellet). 

These days, the majority of feeders are gravimetric – that is, they are designed to weigh and dispense a specified weight of material. Gravimetric feeders incorporate a load cell that automatically weighs the throughput of an ingredient, totaling the loss-in-weight (feeder weight – ingredient weight) as the material is dispensed. These units calibrate automatically and can measure ingredient weights very precisely, to within fractions of a gram.  

For ingredient rates above 1 lb./hr., gravimetric feeding solutions usually work very well. For low and ultra-low feeding challenges, it is sometimes best to move to a volumetric feeding solution. Volumetric feeding delivers a specific volume of ingredient into a process. So, for example, to deliver a low rate (e.g. several grams) of an ingredient in pelletized form, consider using a volumetric system that can deliver individual pellet(s) to achieve the timing and accuracy needed.   

Types of Feeding/Dosing Tools

Screw/Auger

The most common type of feeder today is a gravimetric, loss–in–weight feeder equipped with a screw-type auger. The infeed end of the auger extends into an ingredient hopper, where the ingredient is captured in the flights of the rotating auger and pushed down inside the surrounding cylinder toward the feed throat of the processing machine. (NOTE: Rotating augers tend to collect material somewhat unevenly, leading them to “pulse” out ingredients unevenly—a flow of material with an occasional break. When material/ingredient throughputs are high, or for continuous feeding processes such as extrusion, the impact of slight pulses or breaks is usually minimal. However, when dosages must be dispensed precisely at lower rates – i.e., small shot sizes for injection molding – pulsing can have a negative impact.)  

Cylinder

A dispensing cylinder is offered as an option (in place of a screw/auger) on feeders, including the Conair TrueFeed™ Series. Cylinders rely on slope, rotation, and gravity to produce an even, stream-like flow of ingredients into the feed throat. The smooth, gently rotating cylinder surface virtually eliminates the risk of pulsing or clumping while ensuring that all but the very lowest doses are delivered evenly. Cylindrical feeders can deliver virtually any type of pelletized material, but they really shine when it comes to flowing micropellets or powders. (NOTE: Compare auger vs. cylinder dispensing below.) 

Comparing auger and cylindrical feeding – this illustration shows the difference in accuracy (in grams/shot) between a typical auger, controlled by a DC motor, and a dispensing cylinder controlled by a stepper motor. The peaks and valleys in the auger’s ingredient delivery, compared to the very small variations in the cylinder delivery, represent excess ingredient usage and lost profit. 
Optical Disk

Unlike augers and cylinders, which rely on gravimetric loss-in-weight measurement to ensure dosing accuracy, optical disk dispensers, like the patented TrueFeed™ Micro Feeder rely on volumetric feeding to deliver ultra-low dosages, equal to 5 pellets or less per second. Such small dosages are very difficult to measure gravimetrically due to inherent vibration and electrical noise common to processing operations where feeders must operate. Therefore, a vertically rotating disk is used, which features a circular pattern of perforations.  When gentle suction is applied to the opposite side of the rotating disk, the perforations capture and hold individual pellets. As rotation continues, an optical device confirms the capture of each pellet on the disk. The pellet is then released at the feed throat.  

3. Track Dosing Accuracy

Step 3 is the ultimate test of accuracy: tracking the actual usage of the ingredient in the dosing process. Only this can answer the question: Is the dosing tool accurate to the specified rate?  

As important as this question is, many may not bother. Accuracy seems to be the #1 concern when buyers are shopping for dosing equipment – but many buyers may not take the time to verify feeding accuracy during equipment use. The key reason is that verification requires some data collection and monitoring, and older dosers/feeders can only provide after-the-fact reports about how they’ve performed. Of course, after-the-fact reports don’t alert you to real-time performance problems, such as an operator equipment adjustment or reset, a conveying or loading problem that halts ingredient infeed, or even a failure within the equipment itself.  

To usefully verify dosing performance, you need instant, real-time notification of feeding performance, including readouts of calculated vs. actual equipment outputs. Such monitoring, delivered through controls like Conair’s TrueFeed™ Touch Control, enables you to verify that feeding is working as planned, while informing you of any potential concerns through timely warnings and alarms.  

When time or resources are short, it can be easy to rely on after-the-fact reports, but these make it easy to miss a problem. Then, there’s a tendency to begin “playing it safe” and keeping ingredient dosing on the high side of the spec. Better too much than too little, right? 

But with the right equipment and the help of real-time data collection and monitoring, the opposite is true. If the goal is to use the minimum amount to meet the specification, only data collection and verified accuracy give you the confidence to run that way.   

A Few Quick Rules of Thumb

  • For high and medium rates, a gravimetric loss-in-weight approach works very well. Current gravimetric feeders can measure with an accuracy of 0.5 g/second
  • As the dosage size is reduced, the screw/auger size should be reduced.  As a rule, select the smallest diameter screw/auger that can deliver the desired dose. This is the best way to minimize or eliminate the risk of pulsing/clumping.  
  • When you move into lower rates, 1 lb./hr. and less, even the smallest screw/auger will pulse. So, it’s time to shift to a cylindrical feeder that will eliminate pulsing. This is especially important for small-shot injection molding applications.  

If you have feeding/dosing questions and need answers, Conair experts are available to help.  Give us a call at 800-458-1960.